1. Field of the Invention
The present invention relates to a bearing apparatus. In particular, the present invention relates to a bearing apparatus installed in a blower fan.
2. Description of the Related Art
In recent years, electronic devices have been becoming more and more densely packed with components, and electronic components installed in the electronic devices and blower fans arranged to cool the electronic components have accordingly tended to be disposed close to each other. Such a blower fan is arranged to produce air currents through rotation of an impeller, i.e., a rotating body. In addition, the amount of heat generated in the electronic devices has been increasing year after year, and there has been a demand for an increase in rotation speed of the blower fans. The increase in the rotation speed of the blower fans leads to an increase in a peak value of vibration in each frequency, and then vibrations may exert harmful effects on the electronic components.
Therefore, in order to reduce vibrations which accompany the rotation of the blower fan, it is necessary to reduce oscillation of an axis of a rotating body of the blower fan and axial play of the rotating body. One specific method of achieving this is to adopt a fluid dynamic bearing as a bearing portion to support a circumference of a shaft through a lubricating oil so that vibrations generated in the rotating body can be attenuated. In addition, use of a thrust bearing will contribute to preventing tilting of the shaft. A bearing as described above is disclosed in JP-UM-B 06-31199.
In a brushless fan motor of a type illustrated in JP-UM-B 06-31199, a sleeve is fitted and thereby fixed in a central hole of an inner tubular portion of a case, and a stator is arranged on an outer circumference of the inner tubular portion. In addition, an annular member is fitted and thereby fixed to a lower end portion of a shaft. A thrust bearing is defined between a lower end surface of the sleeve and the annular member, with an axial gap defined therebetween. A radial dynamic pressure bearing is defined between the shaft and the sleeve on an upper side of the thrust bearing. The fan motor described in JP-UM-B 06-31199 has a problem in that dust and the like can easily enter into a gap defined between the sleeve and a combination of the shaft and the annular member through upper and lower opening ends of the gap.
In addition, a fluid dynamic bearing described in JP-UM-B 06-31199 has a problem in that it is difficult to maintain a high precision in axial position of the annular member fixed to the shaft, which may lead to permitting a variation in axial play of the bearing.
There is also a demand for a decrease in the outside diameter of the shaft of the motor in order to reduce a shaft loss through the bearing. Further, there is a demand for an increase in the diameter of the stator in order to obtain a high torque of the motor. In order to achieve both the reduction in the shaft loss and the high torque described above, it is necessary to arrange a bushing between the bearing portion and the stator. When the bushing is used, it is necessary to improve strength with which the bushing and a mounting plate are fixed to each other, and also to increase precision in positioning each of the stator and the mounting plate with respect to the bushing.